In the drilling of boreholes through subterranean earth formations there are various applications in which is is desirable to control certain downhole parameters during the drilling operation. On such drilling parameter is the direction of the borehole as it is advanced through the earth's crust. Often times, if the borehole does not follow the desired course, it is necessary to deflect the borehole in the desired direction. This problem is often encountered in the coal industry where it is a common expedient to form long, generally horizontal boreholes in coal-bearing formations. These boreholes provide for the degasification of the coal formations prior to conducting mining activities. After drilling the boreholes, which are generally horizontal, i.e. generally follow the dip of the coal formation, they are vented to a suitable disposal or collection facility in order to remove hydrocarbon gas (methane) from the coal bed. It is usually desirable to drill relatively long gas-relief boreholes since the methane flow rates are directly proportional to their lengths. One difficulty encountered in drilling such holes is in maintaining the bit trajectory within the desired confines of the coal bed.
Among the systems employed in drilling gas-relief boreholes are rotary drilling systems in which the drill bit is driven by means of a rotating drill string which is connected to a suitable power source located externally of the hole. Such rotary drilling systems are similar in operation to the rotary drilling rigs employed in the drilling of oil and gas wells except that the coal drilling systems are, of course, much smaller and the drill string is rotated by a drill head incorporating a power swivel rather than by a rotary-table structure.
The use of rotary drill string systems to drill gas relief holes is disclosed in Cervik, Joseph, et al. "Rotary Drilling Holes in Coal Beds for Degasification", Bureau of Mines Report of Investigation 8097, U.S. Department of the Interior, 1975. This report describes the use of various centralizer (stabilizer) configurations in combination with bit thrust and bit rotational velocity to control the bit trajectory. Thus, the authors disclose that by placing a short centralizer immediately behind the bit, the hole will follow a slight upward arc under the appropriate conditions of thrust on the drill string and rotational speed. By locating the centralizer about 10 feet behind the bit, the drill string follows a curved path downward. Cervik et al. also disclose the use of two centralizers, one directly behind the bit and the other spaced 10 to 20 feet behind the first centralizer. In this case, the borehole also follows a slightly downward path.
Another system for guiding the advance of a rotary drill bit along a designated path which is particularly well suited to the drilling of gas-relief holes in coal-mining operations, is disclosed in U.S. patent application Ser. No. 371,098 entitled "Method and Apparatus or Rotary Drill Guidance" filed of even date herewith by Emrys H. Jones and Ronald W. Umphrey. As disclosed in the Jones and Umphrey application, the advance of the drill bit is guided along the desired path by repeatedly deflecting the drill string from its axis in a constant radial direction during rotation thereof. Deflection of the drill string is accomplished by the operation of deflector pads located in a segment of the drill pipe. The deflector pads are repeatedly projected and retracted in response to the flow of drilling fluid through the drill string in a pulsating mode which is synchronized with the rotation of the drill string.